Re: [PATCH] mm: fix a data race in put_page()

[Qian Cai <cai@lca.pw>]

> On Feb 8, 2020, at 8:44 PM, John Hubbard <jhubbard@nvidia.com> wrote:
> 
> So it looks like we're probably stuck with having to annotate the code. Given
> that, there is a balance between how many macros, and how much commenting. For
> example, if there is a single macro (data_race, for example), then we'll need to
> add comments for the various cases, explaining which data_race situation is 
> happening.

On the other hand, it is perfect fine of not commenting on each data_race() that most of times, people could run git blame to learn more details. Actually, no maintainers from various of subsystems asked for commenting so far.

> 
> That's still true, but to a lesser extent if more macros are added. In this case,
> I suspect that READ_BITS() makes the commenting easier and shorter. So I'd tentatively
> lead towards adding it, but what do others on the list think?

Even read bits could be dangerous from data races and confusing at best, so I am not really sure what the value of introducing this new macro. People who like to understand it correctly still need to read the commit logs.

This flags->zonenum is such a special case that I don’t really see it regularly for the last few weeks digging KCSAN reports, so even if it is worth adding READ_BITS(), there are more equally important macros need to be added together to be useful initially. For example, HARMLESS_COUNTERS(), READ_SINGLE_BIT(), READ_IMMUTATABLE_BITS() etc which Linus said exactly wanted to avoid.